The concept of a lithium ion battery (LiB) was set in 1962. Right after that, a lithium ion battery (LiB) was suggested by M. S. Whittingham of Exxon Company, leading to the invention of Li—TiS2 batteries. However, commercialization of a battery system using lithium metal and Li—TiS2 as an anode and cathode, respectively, was failed. This is because the anode of lithium metal (LiM) has poor safety and air/water sensitive Li—TiS2 requires high production cost.
Then, the above-mentioned problem was solved by using graphite capable of reversible lithium intercalation/deintercalation and anodic oxide (developed by J. O. Bosenhard) as an anode and cathode, respectively. Therefore, the currently available LiB was successful in commercialization. In 1991, the first commercialized product of LiB was launched by Sony and Asahi Chemicals and brought a progressive chance for leading successful spread of the market of portable electronic instruments. Then, LiB has been used widely and explosively. Particularly, LiB has satisfied a need for electric energy directly related with continuous innovation of general electric devices, such as cellular phones, music players, speakers, drones, vehicles and microsensors. Many researchers and scientists have studied and investigated novel and advanced energy materials, chemistry and physics about the fixed/mobile energy storage systems satisfying an increasing need for energy.
Since the development of commercialized LiB technology reaches a saturated state recently so that gradual improvement of the electrochemical performance of LiB may be reported merely, research and development of a novel energy material having a different shape and composition is required essentially in order to meet the energy requirement. Therefore, secondary batteries having high energy density, such as lithium-sulfur batteries and lithium-air batteries including a LiM anode and a conversion based cathode, have been given many attentions as next-generation batteries.
The cathode based on sulfur and carbon theoretically has an energy density of about 2,600 Wh/kg and about 11,400 Wh/kg, respectively. Thus, it shows an energy density approximately 7 times higher than the energy density (about 360 Wh/kg, C/Co2O4) of LiB. One of the anode materials, LiM, has a high theoretical energy density of about 3,560 Wh/kg as well as a significantly low redox potential (−3.04V Vs. S.H.E.) and a density of 0.59 g/cm3. On the contrary, a graphite anode material has a theoretical energy density of about 372 mAh/g and a slightly high redox potential and density. Therefore, when a graphite anode is substituted with a lithium metal anode, the gravimetric energy density of the battery system improves significantly. When lithium-sulfur and lithium-air batteries are commercialized in the future, it is expected that such a LiM anode and conversion based cathode suggest a hopeful way in overcoming a need for high energy density.
Although such a lithium-sulfur battery using LiM as an anode has some advantages, there are problems in commercialization thereof. First, sulfur has a low electric/ionic-conductivity (5×10−30 Scm−1, room temperature) and the product thereof, Li2S, also is an insulator. In addition, sulfur has a volume increased by about 80% upon a completely discharged state. The final reaction product, Li2S, forms intermediate species called lithium polysulfides (LiPS, Li2Sn, 2<n<8). LiPS are dissolved into an organic electrolyte to cause the problems of loss of an active material and degradation of the electrodes. When LiPS is present in the electrolyte, it moves through the pores of a separator via concentration gradient and arrives at a lithium anode, thereby forming an internal shuttling pathways between the lithium anode and a sulfur cathode. Such a phenomenon is well known as LiPS shuttling. During shuttling, dissolved LiPS, particular LiPS having a high n value, is reduced on the lithium surface and thus passivates the anode surface, resulting in a rapid decrease in capacity, Coulombic efficiency, and the cycle life of the Li—S battery. Although, it is known that LiNO3 additive is effective for increasing LiM. However, it does not provide a perfect solution to protect highly reactive and electrochemically unstable lithium metal anode.
Another attempt is made by ensuring reversibility of electrodeposition of lithium during charging/discharging. Highly reactive and non-uniform electrodeposition of lithium causes problems, such as thermal runaway, decomposition of electrolyte and loss of lithium. Non-uniform electrodeposition of lithium ions that occurs during charging causes formation of Li dendrites that pierce through a separator. Such a short-circuit causes a thermal runaway, leading to a severe safety issue of catching a fire by ignition of the electrolyte. Another problem of LiM batteries includes side reactions of electrolyte and instability of Coulomb efficiency, which makes the battery system inefficient. Such instability occurs due to a continuous reaction among Li, active species, and electrolyte. Thus, solid-electrolyte interphase (SEI) is continuously regenerated, and the electrodes are passivated during repeated charge/discharge cycles. Such an undesired side reactions pile up inactive species at the electrode/electrolyte interfaces in the battery system, resulting in deterioration of the performance of the battery. Therefore, it is necessary to form stable SEIs and to electrochemically and physically protect the lithium surface.
Although the initial researchers have tried to improve the performance of the battery via several means, such as mechanical ball milling of sulfur and carbon or surface coating using carbon, there was no significant effect. To solve the problem of limitation in electrochemical reaction caused by electroconductivity, it is required to reduce the particle size to a size of several tens of nanometers or less or to carry out surface treatment with a conductive materials. For this purpose, there have been suggested several physical methods (melt impregnation into nano-sized porous carbon nanostructure or metal oxide structure), mechanical method (high-energy ball milling), or the like.
In addition, there is a method of forming a coating layer on the surface of cathode particles to prevent dissolution of LiPS or adding a porous material capable of capturing dissolved LiPS. Typically, there have been suggested a method of coating the surface of a cathode structure containing sulfur particles with a conductive polymer, a method of coating the surface of a cathode structure with a lithium ion conductive metal oxide, a method of adding a porous metal oxide having a large specific surface area and large pores and capable of absorbing a large amount of LiPS to a cathode, a method of attaching a functional group capable of adsorbing LiPS to the surface of a carbon structure, or a method of surrounding sulfur particles by using graphene oxide or the like.
Further, a multi-functional separator using carbon and a metal oxide has been applied recently to solve the above-mentioned problem of a lithium-sulfur battery. The use of such a separator using the coating, known as a upper current collector, intercepts LiPS so that it reutilize the active species during charge/discharge cycles, thereby providing improved cycling performance. In addition, the self-discharge property and Coulomb efficiency of a lithium-sulfur batter are improved. However, coating on a separator causes an additional increase in weight of a battery, resulting in a decrease in amount of a cathode material. Thus, it is important to select an adequate coating technology and a proper coating material.